Method for driving a switch in a power factor correction circuit and drive circuit.
The present invention relates to a method for driving a switch controlling the current consumption of an inductive energy storage element in a switching converter that is used as a power factor correction circuit (Power Factor Controller, PFC) and is formed in particular as a step-up converter.
Such a switching converter used as a PFC is sufficiently known and described for example in DE 100 40 411 A1.
In order to afford a better understanding of the problem on which the invention is based, the fundamental construction of such a switching converter is explained below with reference to FIG. 1.
A switching converter used as a PFC has the task of making available a DC voltage Uout for a load from an AC voltage Un, the average current consumption of the PFC, starting from a predetermined power consumption, being intended to be at least approximately proportional to the profile of the input voltage Un in order principally to take up active power.
The switching converter illustrated in FIG. 1 comprises input terminals for applying an input voltage Un, for example a sinusoidal power supply voltage, and a rectifier GL that is connected downstream of the input terminals and provides a rectified voltage Uin from the input voltage Un. Connected in parallel with output terminals of the rectifier is a series circuit comprising an inductive energy storage element L1, for example a storage inductor, and a switch SW, formed for example as a power transistor. Connected in parallel with the switch SW, or in series with the inductive energy storage element L1 with the switch SW open, is a second rectifier arrangement, which, in the example, has a diode D and a capacitor C and also output terminals AK1, AK2 for providing the output voltage Uout.
In the case of this switching converter formed as a step-up converter, the inductive energy storage element L1 takes up energy with the switch closed and outputs said energy to the output capacitor C or the output terminals AK1, AK2 when the switch is subsequently open.
In order to achieve an average current consumption Iin that is approximately proportional to the input voltage Uin, it is known to switch on the switch SW in successive drive cycles for identical switch-on durations ton in each case and then to switch on the switch again after opening, as soon as the inductive energy storage element L1 is free of energy or demagnetized. The temporal profile of the input current Iin, which corresponds qualitatively to the temporal profile of the magnetization B of the energy storage element, is illustrated in FIG. 2a for this mode of operation. FIG. 2b shows the associated temporal profile of the drive signal S20 of the switch SW, which is generated from a control signal S10 and a magnetization signal S21 by a drive circuit 20. The magnetization signal S21 is provided for example by an auxiliary winding coupled inductively to the storage inductor L1. The control signal S10 determining the power consumption is generated from the output voltage Uout by a controller 10.
In the case of the drive method illustrated with reference to FIG. 2, the following holds true for an average current consumption Im per drive cycle of the duration T:Im=Îin/2  (1)where Îin represents the peak value of the input current Iin per drive cycle. Assuming that the inductor L1 is not operated in the saturation region, the following holds true:Îin=(Uin/L1)·ton  (2)where Uin in (2) denotes the instantaneous value of the rectified input voltage Uin and L1 denotes the inductance value of the storage inductor. Taking account of (1) and assuming that the control signal S10— and thus the switch-on duration ton—and also the input voltage Uin change slowly in comparison with the duration of a drive cycle, the average current consumption Im per drive cycle is proportional to the instantaneous value of the input voltage Uin. The average current consumption Im thus likewise has a periodic profile in accordance with the input voltage Uin. The same holds true for the instantaneous power consumption of the switching converter.
Assuming that the input voltage Uin changes very slowly in comparison with the duration of a drive cycle, so that the input voltage Uin may be assumed to be constant at least for a number of successive drive cycles, the following holds true for the power consumption during a drive cycle:Pin=Uin·Im=(Uin2/L1)·ton  (3)
The switch-off duration toff, which corresponds to the demagnetization duration, is proportional to the switch-on duration ton, where the following holds true:toff=ton·Uin/(Uout−Uin)  (4)
Thus, the duration T=ton+toff of a drive cycle is proportional to the switch-on duration ton, and the instantaneous power consumption is thus inversely proportional to the switching frequency f=1/T, said switching frequency varying when considered over a period of the input voltage Uin.
If the switch-on duration ton decreases in order, when considered over a period of the input voltage Uin, to achieve a decreasing average power consumption, then the switching frequency rises overall. In the case of low power consumptions, very high switching frequencies may occur as a result, which is undesirable owing to the switching losses which rise as a result.
A similar problem of a rising switching frequency as the power consumption decreases arises in the case of DC voltage switching converters operated in free-running fashion, so-called DC—DC converters. One possibility for limiting an excessively great rise in the switching frequency in the case of a low power consumption is described for such DC-DC converters in DE 44 37 459 C1, DE 197 32 169 A1 or DE 199 39 389 A1. In this case, after a magnetization of a primary coil of a transformer by application of an input voltage and a subsequent demagnetization of the primary coil, one or a plurality of so-called free transformer oscillations are permitted before the primary coil is remagnetized.
In the case of the step-up converters used as a PFC, such an oscillation after a demagnetization of the storage inductor has a very high frequency and decays rapidly, so that said oscillation cannot be detected accurately enough. Moreover, a waiting time dependent on the frequency of said oscillation would have the effect that the required proportionality between input voltage and current consumption would no longer be reliably ensured.
DE 100 40 411 A1 discloses a switching converter which is used as a PFC and in which the switching frequency of a switch controlling the current consumption of a storage inductor is provided by means of a voltage controlled oscillator in a manner dependent on a difference between the output voltage and the instantaneous value of the rectified input voltage. In the case of this method, the switching frequency follows the profile of the input voltage, which is also the case with a PFC operated in free-running fashion, but, in contrast to a PFC operated in free-running fashion, is not dependent on the power consumption. In the case of this method, pauses which are proportional to the switch-on duration plus the demagnetization duration are established between the demagnetization of a storage inductor and the switching on again of a switch connected in series with the storage inductor. What is disadvantageous about this method is that, in order to provide a signal that prescribes the switch-on instants, the instantaneous value of the input voltage and the output voltage has to be detected as exactly as possible by means of resistor dividers, which is complicated and thus cost-intensive. Moreover, in the case of this method as well, very short switch-on durations may occur in the case of a low power consumption.
A method for driving a PFC in trapezoidal current operation, which does not involve waiting for a demagnetization of the storage inductor before a process of switching on again, is described in U.S. Pat. No. 5,146,398.
It is an aim of the present invention to make available a method for driving a switch controlling the current consumption of an inductive energy storage element in a switching converter, in particular in a switching converter used as a PFC, which limits a rise in the switching frequency as the power consumption decreases, and which can furthermore be realized simply and cost-effectively. It is additionally an aim of the invention to make available a drive circuit that realizes such a method for a switch in a switching converter.